I was cleaning my computer when I came across with those plans for
a piston gun:
I'm guessing that the grey part is the piston and the ratio between the barrel and the comber is 1:2.
Do you think it will work?

The piston valve won't work that way. It needs to seal onto a porting.
This is just a T with a piston in it, not a piston valve.
Check out the "piston valves explained visually" thread for more information on how piston valves work.

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Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!

Only if you are using a shrader to fill.
If you are using a quick connect, you'll have to close it before disconnecting. However, you could vent through the quick connect, and leaving the top BV out.
Saves a lot of pilot volume. (T and two elbows)

Kinda old thread you dug up though.

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Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!